so i've been a kamen rider fan since decade and i thought his power was really cool and unique at the time being able to use the form and powers of past riders but over time something happened, Decade became less unique and special.

it started with kamen rider gaim in that series a few of the riders (Gaim, Baron, Ryugen and Zangetsu ) in a movie gained legend rider lockseeds
that granted them the powers and weapons of past riders the in the gaim & drive teamup movie they did it again by giving gaim a drive lockseed and giving drive a gaim shiftcar,

then a year later in kamen rider ghost did the same thing by giving the riders legend rider eyecons, and then in the ghost ex-aid teamup movie they gave ex-aid a ghost gashats.

and then there's kamen rider build who also go figure gained legend rider powers but unlike the previous riders who's legend rider suits fitted into there own series build does what decade did and takes the form of the rider 's power he's using.

and now on to kamen rider Z-IO who just like the other riders i mentioned can use the powers of past riders using rider watches that gives them access to rider armors.

but this "taking the forms and powers of past heroes" thing would not be something only kamen rider would do because two years after kamen rider decade we got Kaizoku Sentai Gokaiger and the gimmick of that show was the sentai team could use the powers and forms of past sentai teams. hmm now where have i heard that before!

lets not forget the time the gokaigers used the forms of kamen rider OOO's and the forms of metal heroes!

so in closing kamen rider decade's ability to use the powers of past riders was something that didn't stay his for to long.

One could argue that Decade was never special. With his sole defining trait being able to transform into other riders, and all of his powers stem from them, he never developed an identity or power of his own. The Final Form rides are reliant on another Rider being present to even activate, and even then, it's just a big weapon.

Post Decade Riders getting the legacy items, I'd argue, actually do it better than Decade. Gaim Kuuga Arms doesn't just turn into Kuuga with Gaim's belt, but takes the lockseed gimmick and creates a themed armor designed like Kuuga, for example.

Zi-O takes this idea, and implements it in the series, rather than just having them be one off movie/special transformations that never show up again, by having a central gimmick that is themed around the past riders.

That was a point Decade had, though -- one made absolutely explicit in his Movie War with W. That he was a wandering traveller with no story of his own; someone who was only haphazardly shoved into the stories of others. His entire role and identity changing with each world was largely played for laughs, but it still very much illuminated the point that that was because he had nothing of his own and was only made more pressing when his companions didn't change alongside him.

... granted, it could have stood to be better. Tsukasa and Natsumi for me are characters that absolutely save a season that was otherwise a bit too confusing and perhaps thought a bit too highly of itself and its concept; but said concept being a metacontextual look at what exactly Kamen Rider was at the time still manages to be an intriguing one.

Gaim and Zi-O take their predecessors' power as armour. Ghost inherits their souls. Drive uses their skills and Ex-Aid proves once again that he really likes video games. But Decade only has a bootlegged version of the literal rider form itself rather than any derivation or anything to make it truly his; hell, the Decadriver itself is modeled after previous belts and is nothing more than a proxy to bypass the abilities of other drivers.

Decade simply uses others' powers without what made them special; has nothing of his own that makes them special; and is himself not special. And that's the point.